1. Field of the Invention
The present invention relates generally to a current type GTO (gate turn-off thyristor) inverter, and more specifically to a surge voltage clamping circuit for clamping the surge voltage generated when each GTO incorporated in a current type GTO bridge-connected inverter is turned off. The clamped surge voltage is stored once in a capacitor and then returned to the terminals between the rectifier and the GTO inverter for energy restoration.
2. Description of the Prior Art
In a current-type gate turn-off thyristor bridge-connected inverter, since gate turn-off thyristors (referred to as GTOs, simply hereinafter) are incorporated in the inverter as the main switching elements, no commutating circuit including a commutation reactor, for instance, is required, because the GTO can be turned from on to off or vice versa in response to a control signal applied to the gate terminal thereof. Here, the terminology "commutation" means that the load current of one phase is switched to that of another plane or vice versa by thyristor switching operation. In the above-mentioned current-type GTO inverter, however, in the case where a load such as an induction motor having an inductance is coupled, commutation surge voltages are inevitably generated whenever each GTO is turned off. The generated surge voltages are superimposed upon the alternating output voltage of the GTO inverter, thus resulting in a problem in that some of the GTOs may be damaged by these commutation surge voltages.
In order to overcome the above problem, a commutation surge voltage clamping circuit has been proposed, by which the commutation surge voltages generated whenever each GTO is turned off are absorbed or stored in a single electrolytic capacitor and thereafter returned to the load side through the GTO inverter for reducing the electric power loss. This function is called energy restoration.
In the conventional commutation surge voltage clamping circuit used for a current type GTO inverter, however, there exist some disadvantages as follows.
(1) Since a pair of ordinary thyristors are used for restoring the stored commutation surge voltage energy to the DC source terminals of the GTO inverter, two vibration circuits or thyristor turning-off circuits including a capacitor and an inductor respectively are necessary. Further, since the surge voltage energy is restored through these capacitors used for the vibration circuits, the capacity of these capacitors of the vibration circuits is determined to be relatively large. As a result, the turn-off operation of the ordinary thyristors often fails at higher frequency range. In other words, it is impossible to stably operate the commutation surge voltage clamping circuit when the GTO inverter operates at a high speed.
(2) Since the commutation surge voltage energy is restored from the electrolytic capacitor to the DC source terminals of the GTO inverter through the vibration capacitors connected in series with the electrolytic capacitor, the capacitance of the restoring circuit is relatively large. Therefore, a reactor having a large inductance is necessary in order to smooth the current restored to the GTO inverter. In other words, the cost of the commutation surge voltage clamping circuit is relatively high.
(3) Since the charging and discharging circuits of the capacitor are operable only when the motor is driven in the forward or the reverse direction, when the motor is being braked, it is impossible to regenerate the motor kinetic energy stored in the capacitor in motor-braking operation to the AC source side of the inverter or to charge the magnetic energy stored in the reactor in motor-braking operation in the capacitor.
A more detailed description of the prior-art commutation surge voltage clamping circuit will be made with reference to the attached drawings under DESCRIPTION OF THE PREFERRED EMBODIMENT.